The predominant glazing material used by operators of urban transit systems is tempered or laminated glass. Where breakage is a concern, more expensive plastic glazing materials such as polycarbonate sheet are used because it is as clear as glass and virtually unbreakable. The cost of polycarbonate glazing may be as much as a factor of four or five times greater than the cost of similar gauges of glass. While plastic glazing is generally less breakable than glass glazing, it is more susceptible to being defaced by scratches and vandalism than glass glazing. Because the scratches are hard to disguise and when disguised interfere with visibility, scratched or vandalized plastic glazing must be replaced. Use of glass based glazing laminates for transportation applications is not an acceptable alternative insofar as when the glass glazing is broken, the bus or train wherein the glazing material is so damaged must be immediately pulled from service for replacement and repair of the damaged window or glazing material. Thus issues of public safety and convenience militate for less breakable glazing materials.
Glazing laminates are known and used in a variety of applications. One example is automobile safety glass which has typically comprised two layers or sheets of glass separated by an interlayer of a tough transparent plastic such as a plasticized polyvinylacetal. Laminates comprising polycarbonate sheets in contact with plasticized polyvinylacetal sheets are described in U.S. Pat. Nos. 3,406,086,3,539,442, and 4,128,694. A more recent U.S. Pat. No. 5,126,208 describes a non-symmetrical glass plastic laminate wherein the plastic may be polycarbonate or polyacrylate. Thus the laminates may either be symmetrical or unsymmetrical about the substrate which may be any one of several optical materials such as glass, polycarbonate, polyacrylate, and the like. The materials being laminated to the substrate must possess both acceptable optical properties and durability and these criteria must also apply to the adhesives used to effect the bonds between the laminar layers.